Experimental investigation of the sub-barrier fusion of neutron-rich light nuclei is important in understanding the crusts of neutron stars, the structure of neutron-rich nuclei, and fusion dynamics of neutron-rich nuclei. The fusion of very neutron rich nuclei such as 24O+24O and the depth at which they occur impacts the temperature profile of the star and consequently other observational properties. Two examples in which such fusion heating may be important involve the initiation of X-ray superbursts and the cooling rate of crusts following X-ray bursts. Radioactive beam facilities at GANIL and MSU-NSCL now make such measurements, which are important for both nuclear physics and nuclear astrophysics, feasible.
In collaboration with the Nuclear group at the University of Notre Dame, we are planning to measure the sub-barrier fusion cross-section with stable beams using the new high intensity Notre Dame tandem. This high intensity machine which has been installed and is presently being commissioned provides the opportunity to make significant improvements in sub-barrier fusion studies. Our interest is two-fold. We are both specifically interested in the 12C + 12C sub-barrier measurement, determining the cross-section at sub-barrier energies and establishing whether sub-threshold resonances exist. The suppression of the fusion cross-section for this reaction in the deep sub-barrier domain is a topic of considerable debate. We are also interested in the overall behavior of sub-barrier fusion. Such measurements are complementary to our fusion studies with neutron-rich beams. In understanding potential fusion reactions in the crust of a neutron star, i.e. at astrophysically relevant energies, it is necessary to understand the dependence of the astrophysical S factor on both neutron number and energy.
When two heavy-ions collide at intermediate energies, partial overlap of the
two nuclei in a peripheral collision can lead to copious production of
clusters at velocities intermediate between the projectile and target. These
fragments are not produced as a result of statistical decay of an equilibrated system but are dynamically produced -- the process resembles a
"tearing". It is well characterized by the alignment, kinetic energies,
and sizes of the fragments, however the mechanism is not understood.
Of course the system involved is a microscopic one! We now have access to the equilibration time scale of the N/Z degree of freedom by studying those collisions, which is of high importance in establishing the Equation-Of-State of Nuclear Matter.
Fission provides unique conditions to study the interplay of nuclear structure and dynamics.
Investigating fission is relevant in examining the structure of nuclei at extreme deformation,
as well as in establishing the limits of existence for superheavy elements. As a heavy nucleus undergoes
fission either spontaneously or at low excitation energy, shell structure strongly influences
the probability of fission as well as the mass asymmetry of the fission products. These observables
are thus a manifestation of the quantal nature of the finite, strongly interacting nuclear system.
Consequently, their proper description represents a fundamental challenge for nuclear models.
Examining fission as a function of N/Z allows us to examine how the surface and shape
degrees-of-freedom evolve as a nucleus becomes more neutron-deficient.